cob vs earth bags

nicole schalk

Posts: 2

posted 1 year ago

hello,
My husband and I are wanting to build a house and we have been dreaming of a cob house for years. He wants to move to CO and we are wondering if earth bags will be better in the winter than cob. we do not really want to do straw bale. can we still cover earth bags with cob and earth plaster?
do the earth bags retain heat in the summer? :/

Depending on where in Colorado you are, I anticipate you're going to want some form of insulation.

The idea of thermal mass replacing insulation can work, but only when it's rather warm every day and rather cold every night.

For example, here in Michigan, fuggedabaddit. We often go weeks in the winter with no direct sun and daytime highs in the teens Fahrenheit. Uninsulated walls, even if massive, will just be COLD.

That's one reason why there's no tradition of adobe houses here like there is in the desert SW.

Andrew Parker

pollinator

Posts: 514

Location: Salt Lake Valley, Utah, hardiness zone 6b/7a

4

posted 1 year ago

Much of the Desert SW is on high plateaus or high plains. Santa Fe and Taos get very cold in the winter. I remember driving from Gallup to Cortez on a clear December night at 14 below. The next day, the clear, sunny turquoise-blue sky warmed up to single digits.

A thick cob or adobe wall would be pretty comfortable, with a good stove. Adding insulation would make it cheaper to stay comfortable.

If you are only looking for a cob aesthetic, you can pretty much cover anything with a veneer of cob, including straw bales, straw-clay or woodchip-clay walls -- or a super-insulated wall.

Now would be a good time to travel to where you intend to build and see if you can visit some of the "natural" homes there and ask questions of the residents. Also go in late July through August to see how the same homes deal with the heat.

Also, Colorado has some extremely varied physiographic regions. It would help if you identified which one you are interested in moving to and its elevation.

Your best bet is to select the material(s) that will perform best in your climate and weather conditions.

If you experience long periods of cold and clouds obscuring the sun, a hybrid type of construction might serve you better.
I've seen double wall chord wood homes, with an insulation core, do far better in cold, no sun for days climates.

Cob walls in colder climates work fairly well because those walls tend to be very thick at the base and then taper as they go higher.
When I was a child, we stayed in a cob house during a winter while in England, the fireplace kept the house pretty cozy but the walls were close to 1 meter thick at the ground and around 30 cm thick at the ceiling.

You can use insulating materials (pearlite, vermiculite, etc.) mixed into the fill of earth bags, not so much in cob.
It is very possible to use a double wall construction with both of these methods so you have an interior wall then a wall of insulation and then the exterior wall, this way you get the look you want and great insulation numbers.

In the end it is up to you to determine your needs first then the wants can be added to get the finished look you desire.

A suggestion made by Owen Geiger (at the Earthbag Building Blog) is to fill earthbags with Scoria instead of dirt (as Bryant Redhawk already stated). More expensive, but it will insulate the walls and provide structural support. Then you can cob over the walls. Here's the link:

The earthbag walls are laid and then covered with strawbales on the outside. It merits some consideration, and although I'm sure it could come with its own range of hurdles, it could probably be done on a cob wall as well. If you like super thick, hay-filled walls.

Also, keep in mind that not all the structures possible with cob can be created with earthbags, and vice versa. If you want a long, straight-walled house, you probably shouldn't consider earthbags.

And I'm by no means an expert. I plan to build an earthbag root cellar or greenhouse this summer, but I've never actually done anything with them. But I read a lot.

There are two seasonal diversions that can ease the bite of any winter. One is the January thaw. The other is the seed catalogs. ~Hal Borland

nicole schalk wrote: I didnt know we could do these in michigan. i live near saginaw currently. do you think that earth bags would be better or worse then strawbale?

Strawbale needs a foundation above ground level to keep moisture from wicking into the walls as I understand this construction method.
With strawbales you get insulation from the material without the need for a separate layer of insulation as you would need in earthbag construction.

Earth bags can be started from a packed gravel footing.
You don't need to use needles and some sort of wire mesh to hold everything together with earth bags, just two runs of barbed wire between each course of bags and anchors at the roof supporting structure unions are about all that you must have.

In our world of alternative building techniques, I always recommend lots of study of all available techniques before finalizing the decision of method to use.
There are many variables which mandate expanding your knowledge to the point that you have taken into account; stability needs (earthquake zone?, flood zone?, brush fire zone?, etc.),
amount of man power needed to finish close to your completion date (if there is one), the design of the structure (includes number of floors, roof pitch, number of windows and doors, etc.),
type of site you are building on, local and state building codes/ requirements. Once all these are addressed, then you are ready to start your build.

The most important thing (my opinion only) is that you meet all or at least most of your desires for style, comfort, ease of construction and ease of maintenance down the road.

I have also been thinking of building a home in a cold climate. I posted the following on a different forum; it might give you an idea or at least start some discussion:

The thermal conductivity of earth bags had me concerned at the start. I actually began my deliberations thinking of a modified slipform stone masonry building but changed my mind when I stumbled across a sprayfoam video. They were insulating older buildings and had developed a nondestructive method where the foam was sprayed to the outside of the building and stucco was then applied shortly after. I believe this would stop the thermal conductivity disadvantage that earth bag has in cold climates while enhancing the thermal mass advantage it has. It would be similar to the strawbale suggestion above but replaces the building of two structures (an earthbag home and a strawbale home) with hiring a contractor for a couple of days. While I have concerns with offgassing, this method does not have the foam applied where you will be living and there are some environmentaly friendly foams out there.

Before anyone goes off on me please know that I understand sprayfoam is (somewhat) expensive and/or distasteful to some... A small 15' diameter circle 1.5 stories high I estimate would cost 3k ish to insulate/stucco. I also realize it is not the most Eco minded material but I believe that, in balance, the structure would have a smaller carbon footprint than a conventional build, especially over its lifetime.

Cheers,
Gregory

T Phillips

Posts: 39

Location: Colorado Springs, CO zone 5A / Canon City, CO zone 5B

posted 1 year ago

We completed a scoria bag tank house, 14' in diameter, last November near Canon City, Colorado. Well, we did the bagging, and a shotcrete company finished it inside and out with 3-4" of concrete. The bags were deteriorating so badly that we had no choice if we were going to save the work. The goal of the project was to harness the water from the flowing well that sits 400' downhill from the house, keeping the energy inputs to prevent freezing to a minimum, and to build something that the local mouse population and fire couldn't destroy. So far, so good.

Mike put two temperature sensors to work, one on the outside on the north, and one inside. Before he accidentally fried them with static, we got some interesting results. The first chart is for about 3 weeks with NO energy inputs in January, the second is for 3 weeks with one incandescent 60w bulb on in Jan/Feb. It is not apples to apples, but it is illuminating nonetheless.

We are going to start building a house in the spring, if we ever decide on a wall system. If we were doing bags, we would do a double wall system, earth in the inside row, and scoria or more likely perlite, in the outside row. There is no way in this climate we would ever consider a high thermal mass house without significant insulation. Just my 2 cents.

We have been looking for a wall system that, once completed, needs no finishing work, either inside or out. It is a challenge.

TankTemp1.jpg

TankTemp2.jpg

Greg.E Campbell

Posts: 5

posted 1 year ago

We have been looking for a wall system that, once completed, needs no finishing work, either inside or out. It is a challenge...........what way are you thinking of going

T Phillips

Posts: 39

Location: Colorado Springs, CO zone 5A / Canon City, CO zone 5B

posted 1 year ago

The only serious contender so far is slipform stone, inside and out, with a space of as yet undetermined size for an as yet undetermined insulation. This is a whole lot of material and time. Two 8" minimum rock "faces" and a cavity for pertile or blue board or spray foam. I'll be 60 years old in the fall, and I cannot even comprehend how long it would take me to build even a small house with this, and since I'm claustrophobic, really small isn't an option. Mike is the bread winner, I am the happy labor. I'm not sure how happy I'll be at the end of that project like that. Besides, we have 20 acres to plant stuff on and a greenhouse in the plans, so...

Just for information's sake, it took 2 months of bagging, mostly one person, working 4-6 hours/day, to build the 14' scoria dome. The bags "tamped out" at about 12". They weighed about 25-30 lbs. each, which became after 2 weeks, very do-able. It took a 3 man crew 4 days to do the concrete covering.

Beldar.jpg

Greg.E Campbell

Posts: 5

posted 1 year ago

One of the hardest jobs I have ever done is build walls of stone+cement. The slip form will make it a lot easier,but that is still a helluva job lol enjoy. I did a lot of pigsties+labourers cottages years back using quarry dust(fines) cement stamped into moving shuttering using soda can as a honeycomb insulation but I am in africa, so cold is not a big consideration

T Phillips wrote:We have been looking for a wall system that, once completed, needs no finishing work, either inside or out. It is a challenge.

I'm sure you've looked at cordwood/cobwood walls as part of your search - is there any particular reason that doesn't fit the bill? That's our running plan here in central maine and has proven out as acceptable in all respects in our experimental structures so far. Very nice aesthetic inside and out, if you like that sort of thing, and the insulation in a M-I-M wall can be pretty much as thick as you desire Good fire resistance, too.

Bryant RedHawk wrote: Strawbale needs a foundation above ground level to keep moisture from wicking into the walls as I understand this construction method.
With strawbales you get insulation from the material without the need for a separate layer of insulation as you would need in earthbag construction.

This is true for pretty much any style of construction. I think people would find even concrete walls might benefit from a porous base of gravel to above grade. Certainly cob or earth bag.

It may be that strawbale needs more than "just" gravel under the base row though. (to keep life out?) The info I have is that concrete wicks moisture... I am not sure if that is true or if it just keeps/collects moisture on the inside out of the air. Just that homes with a solid concrete foundation tend to be damper than those with gravel.

T Phillips

Posts: 39

Location: Colorado Springs, CO zone 5A / Canon City, CO zone 5B

posted 1 year ago

Hey Tristan-

Yes, we've looked at cordwood, and it is still in the running, just down on the very short list. Here in Colorado, fire is something that has to be respected big time. My mantra for finding something for the walls was: no bugs, no fire, no rodents, no tornadoes (perhaps more a function of shape than material), no decomposition. We are looking to hand this down through the family for as many generations as it will function properly. so the long term is as important as the short term. Cordwood can be done by one person, although not very efficiently, and that is appealing. I'm a smidgen skeptical about the insulation values, but it's got to be better than the manufactured POS we're in now, or the 2x6 throw-it-up-as-quick-as-possible we just sold in the city.

Part of the trouble with the decision making process is that we are ignorant, and there aren't a whole lot of people to talk to about living in well insulated, low tech, high thermal mass houses except the earthshippers. A lot of people feel the need to defend their choices, so there isn't a lot of objectivity. I suspect I am allowing the phrase, "the perfect is the enemy of the good (or good enough) to cloud my judgement.

I feel as though I have hijacked Nicole's thread. Here are the three earthbag projects I have read about in challenging climates. They might prove illuminating and relevant to a cob build. I don't know the details of the fill, but I believe

I wish you luck, Nicole. I, too, dream of a cob home. Let us know about your thought processes for deciding.

Len Ovens

pollinator

Posts: 1452

Location: Vancouver Island

29

posted 1 year ago

T Phillips wrote: I feel as though I have hijacked Nicole's thread. Here are the three earthbag projects I have read about in challenging climates. They might prove illuminating and relevant to a cob build. I don't know the details of the fill, but I believe

Interesting projects For me they are not that interesting because they spend too much time on the building process and not enough on what living in it after is like. I have seen so many projects where the last pictures one ever sees is the finished (or almost finished) house empty. I always like to see pictures with furniture and nick nacks, places that look lived in say a lot to me. The last site above has another page that to me says a lot more than the link above:

This post answers questions on what they would have done different. It is interesting in both what worked and what could have been better. For the most part it seems to have worked well. I can relate to the climate as I grew up in the area which is helpful to me as well.

Another site that has both during building and after living posts is:
http://www.themudhome.com/ The bags are filled with earth and covered with cob. The home has been through earth quacks, floods, wind storms (180km) and rain. The owner feels it is a safe comfortable home. A little warmer there perhaps, but still good info.